1. Field of the Invention
The present invention relates to an optical transmission controller. More particularly, the present invention relates to an optical transmission controller which has application in the field of optical communications and which employs a semiconductor laser as a light source for optical signal transmission.
2. Description of the Related Art
With increase in information volume and communication speed, optical communications are finding increased use as a means for transmission of information. They are currently used mainly in trunk line systems, but are predicted to have application in apparatus-to-apparatus communications and in networked systems in correspondence with the computerization in offices and households as they are expected to be in widespread use. For the optical communications, there are used optical fibers as a communication medium and transmit/receive apparatuses for transmitting/receiving light.
In the field of communications, there is a strong demand for improvement in communication speed for obtainment of more information in a shorter time, and this also holds in the field of optical communications. The improvement in optical communication speed requires inexpensive light sources that enable high-speed optical modulation. In recent years, a semiconductor laser that enables direct modulation at a speed of about several hundreds of Mbps to 1 Gbps has been often used to meet this requirement. Though in trunk line systems and the like that require a higher modulation speed, external modulators are employed for modulating laser beams, such modulators are not suitable for consumer use because of their complicated construction and expensiveness. Further, there is a common perception that a communication speed as high as 1 Gbps is sufficient for consumer use with the current state of art.
It is desired that plastic fibers be used since they are inexpensive as a medium and excellent in their connectivity to apparatuses for a low-cost construction of optical communication systems. The use of plastic fibers is advantageous in that a red semiconductor laser (operated with λ in the vicinity of 650 nm) is available as an inexpensive light source in the use of plastic fibers since recent years have seen a remarkable proliferation of the red semiconductor laser for use as a DVD device laser.
With regard to the drive of a semiconductor laser through direct modulation, it does not pose any particular difficulty when the modulation speed is about 100 Mbps or lower, while when the modulation speed is several hundreds of Mbps or higher, it presents the following challenges to be solved:
(1) Ensuring the quality of a modulation input signal supplied to a driving circuit for driving a semiconductor laser.
(2) Increasing driving-circuit operation speed.
(3) Contriving circuits and wirings existing between the semiconductor laser and the driving circuit.
Conventional measures to solve the above challenges include:
(1) Adopting a high-speed interface of small amplitude such as one in a PECL (Positive Emitter Coupled Logic) or a LVDS (Low Voltage Differential Signaling) for input to the driving circuit.
(2) Integrating driving circuits and adopting processes for manufacturing a semiconductor laser that is suitable for high-speed operation such as a BiCMOS or a compound semiconductor laser.
(3) Minimizing inductance components that block high-speed transmittance of the signal by matching impedances between the semiconductor laser and the driving circuit and by using wirings that have the shortest possible length.
For example, with regard to the above challenge in item (3), a technique is known about high-speed drive of a semiconductor laser (see, for example, Japanese Unexamined Patent Publication No. Hei 5(1993)-327617).
In conventional methods of driving a semiconductor laser, an oscillation threshold current is constantly provided as a bias current to a semiconductor laser and a modulation current is superposed on the oscillation threshold current for generating modulation light. In recent years, however, there is a demand, with increase in communication speed, for reduction in power consumption of apparatuses. One measure to meet the demand is considered to be to reduce the bias current in driving the semiconductor laser.
The reduction of the bias current is achieved as follows. When the semiconductor laser is driven starting with a bias current not higher than the oscillation threshold value, an oscillation delay occurs in the laser. It is assumed, however, that this raises no problem as long as systems are used in a range permitted by the specifications of the systems or as long as the semiconductor laser is driven so as to compensate the delay in pulse width.
What is specifically problematic when the semiconductor laser is driven starting with a bias current not higher than the oscillation threshold value is that a significant relaxation oscillation occurs in an optical signal output (hereafter, also referred to simply as “an optical output”). In such a case, when a modulation signal frequency is high, a modulation is effected before the relaxation oscillation does not subside yet. At this time, a fall state of a waveform of the optical output (a fall portion of a waveform of the optical output) is subject to the relaxation oscillation.
The relaxation oscillation of the optical output will be briefed with reference to FIG. 5 (see: solid line waveforms in FIG. 5). When a bias current Ib not higher than the oscillation threshold voltage is fed to the semiconductor laser and is raised to a value I at the time t=t0, an injection electron density (injection carrier density) n in the semiconductor laser increases starting with a value nb assumed when the bias current Ib is started to be fed, with a time constant τ n (=lifetime of electron) in accordance with an exponential function. When the injection electron density n reaches a threshold electron density nth upon the lapse of a time td, the optical output from the laser is started and a photon density nph rapidly increases.
Thereafter, the injection electron density n continues to increase beyond the threshold electron density nth, and then turns to decrease with increase in the photon density nph (with increase in light emission). When the injection electron density n decreases to the threshold electron density nth, the photon density nph turns to decrease. Then, the photon density nph becomes sufficiently small when the injection electron density n, which once has decreased to a value not higher than the threshold electron density nth, again turns to increase to reach the threshold electron density nth. The above motion is repeated several times before a steady state is attained. This transient phenomenon is called relaxation oscillation. A frequency fr of this relaxation oscillation is said to be normally several hundreds of MHz to several GHz and is represented approximately by the formula (i):fr=1/(2π)×(1/(τn·τph))1/2×((I−Ith)/Ith)1/2  (i),wherein τ ph is the lifetime of a photon in a resonator.
Next, the time to switch off the input current (input signal) will be examined. In the case where the input current is switched off after the termination of the relaxation oscillation, the fall portion of the waveform of the optical output is not subject to the relaxation oscillation. In the case where the input current is switched off during the relaxation oscillation, however, the fall portion of the optical output waveform is subject to the relaxation oscillation. Referring to FIG. 5, the input current is switched off at times t1, t2, t3 and t4. The times t1, t2, t3 and t4 are four typical times in one variation period of the injection electron density n (one period of the relaxation oscillation). The time t1 is where the injection electron density n turns from increase to decrease, the time t2 is where the injection electron density n is on the decrease at the threshold electron density nth, the time t3 is where the injection electron density n turns from decrease to increase, and the time t4 is where the injection electron density n is on the increase at the threshold electron density nth.
Indicated by waveforms {circle around (1)} to {circle around (4)} of FIG. 5 are ones obtained by switching off the input current (input signal) at the times t1, t2, t3 and t4, respectively. As seen, the fall state of the waveform of the injection electron density n (optical output waveform) varies depending on when the input current is switched off. The waveforms {circle around (3)} and {circle around (4)} compared to the waveforms {circle around (1)} and {circle around (2)} increase the injection electron density n even after the input current is switched off so that light is emitted with the injection electron density n above the threshold electron density nth. In other words, the waveforms {circle around (3)} and {circle around (4)} have further oscillation crests after the input current is switched off. This serves to deteriorate the fall state of the waveform of the optical output and to prolong a fall time thereof. With such optical waveforms, an Eye Opening cannot be ensured in waveforms obtained during communication, and thereby the communication quality is significantly deteriorated. Further, lowering the communication speed becomes necessary for ensuring the communication quality.
As understood from the above, when the semiconductor laser is high-speed driven starting with a value not higher than the oscillation threshold value, the fall portion of the optical output waveform is subject to the relaxation oscillation depending on when the input signal is switched off, and thereby the communication quality is deteriorated and the high-speed communication is impaired.
This problem cannot be coped with by the above mentioned conventional measures.